In the blast furnace for iron making, coke has the role of providing bed permeability to gas as well as serving as a reducing agent and energy source. Therefore, the coke must have the strength to sustain the weight of the burden. However, coke strength is decreased by the reaction C+CO.sub.2 =2CO. This reaction is unavoidable because it produces CO which reduces Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4. However, it results in degrading coke due to oxidation. This degradation leads to coke powdering and this powdered coke hinders the passage of gas through the burden. This is the most undesirable situation for the operation of the blast furnace.
The traditional method of avoiding this problem has been to use high grade coke resistant to oxidation by CO.sub.2. However, coal convertible to high grade coke is far less available than ordinary coal and is expected to be exhausted in the future.
Coke is made from coal in inefficient "coke ovens" which tend to cause air pollution. Because of the three requirements that coke serves as a reducing agent, a fuel supply and as a support, only a fraction of the world's coals make suitable coke and the difficulties in turning them into coke results in coke being a relatively expensive ingredient in making iron (and subsequently, steel). Stringent requirements are imposed with respect to the reactivity of coke and its resistance to degradation in the high temperature oxidizing environments encountered in blast furnaces.
M. Ogawa, M. Miyawaki and T. Tuyuguchi, 113th ISIJ (Iron & Steel Institute Japan) meeting of April, 1987, Lecture No. S-62, discuss the modification of coke to increase the resistance to oxidation with carbon dioxide. Road tar was heated and dropped on coke heated in the furnace. Thermally cracked carbon was deposited within the coke. Deposition of the thermally cracked carbon was found to improve the reactivity (decrease in oxidation rate) and thereby increase the coke's strength. This method is considered to be effective to improve low-rank coke. However, industrial applications appear to have been precluded by the generation of dust during the cracking process.
U.S. Pat. Nos. 3,725,018 and 3,725,019 describe a method of coating the exterior surface of low-grade coke with a film of glanz carbon. The coating minimizes dusting by providing a hard, dense surface which fills pores adjacent to the surface. The coating is formed from hydrocarbon cracking in the presence of a catalyst.
The oxidation of the coke with CO.sub.2 mainly occurs in the lower part of the shaft of the blast furnace where the temperature is 1173.degree.-1773.degree. K. In this temperature region, oxidation occurs on the internal pore surface. The small pores (30 nm&lt;r&lt;0.3 .mu.m) participate in oxidation to a greater extent due to their larger (by one hundred times or more) surface area compared to that of large pores (r&gt;10 .mu.m).